1. Field of the Invention
The present invention relates to hollow particles of crosslinked melamine resin having a uniform particle diameter and to a process for producing the same. The hollow particles are useful as an organic filler.
2. Description of the Prior Art
Hollow particles of acrylic resin are known as shown in "Koubunshi Mikuro-shashinshu, Me de miru koubunshi" (Microphotographs of polymers--visualized polymers), vol. 1, p. 107-108, compiled by Koubunshi Gakkai (Society of Polymer Science), published by Baifukan. However, they are inferior in heat resistance to those of thermosetting resin and hence they are not suitable for use in the application area where heat treatment (such as heat drying) is required.
There are known porous particles of thermosetting resin as disclosed in Japanese Patent Publication No. Sho 63-20455 (20455/1988) and Japanese Patent Laid-open No. Sho 50-122564 (122564/1975). However, there are not known hollow particles of crosslinked thermosetting resin having a uniform particle diameter exist similar products such as microcapsules for pressure-sensitive paper, they need a complex process and present difficulties in keeping the particle diameter uniform.
In addition, there are available solid particles of crosslinked resin having a uniform diameter. However, they are poor as a heat-insulating filler. Moreover, as compared with inorganic pigments, they are also poor as an opacifying pigment to be added to paper or paint because of its refractive index close to that of paper or paint.